1. Field of Invention
The present invention relates to semiconductor lasers, and more particularly to a cooling module for manufacturing a liquid-cooled semiconductor laser, its fabricating method, and a semiconductor laser fabricated from the module.
2. Description of Related Arts
A semiconductor laser is also referred to as the laser diode (LD). Since the eighties of the nineteenth century, technologies in semiconductor physics have been greatly developed. Specifically, new technologies such as the novel structures of the quantum well (QW) and the strained quantum well (SL-QW), refractivity-modulated Bragg transmitter and modulation-enhanced Bragg transmitter, especially, the new techniques of MBE, MOCVD and CBE for the crystal growth and the new technique of epitaxial growth, which can precisely control the crystal growth to the atomic layer thickness, have been developed. By taking advantages of the latest development of technologies, it is possible to grow excellent quantum well materials and strained quantum well materials. As a consequence, the LDs as fabricated had remarkably reduced threshold currency, greatly enhanced conversion efficiency, times-fold increased output power, and apparently elongated lifetime. With the continuing improvements in stability, conversion efficiency and output power of semiconductor lasers, high-power semiconductor lasers have been found in increasingly wider applications in industry, medicine and military, therefore, in great need in the market, and have shown ever broader prospect for future development.
With the continuing and rapid development of the more and more applications of lasers, stricter and stricter requirements, by the various fields, have been raised to high-power semiconductor lasers, requiring them to be further improved in terms of output light power, conversion efficiency, reliability and performance stability. In addition to the chips, the performance of lasers is related to heat dissipation and packaging. In order to improve reliability and performance stability, and to lower production cost of lasers, highly reliable packaging structures and highly effective heat-dissipating structures, as well as simplicity and low cost, have always been pursued in the design and manufacture of semiconductor lasers.
Currently, there are two packaging modes for high-power single-array semiconductor layers, i.e. thermal-conduction cooling type (Michael Leers, Konstantin Boucke, Manfred Gotz, et al., Thermal resistance in dependence of diode laser packages, In: Mark S. Zediker eds. Proceedings of 56 SPIE, 2008. 6876 (687609)) and micro-channel liquid cooling type (Rushikesh M. Patel, David K. Wagner, Allen D. Danner, Kam Fallahpour, Richard S. Stinnett, “Use of micro-channel cooling for high-power two-dimensional laser diode arrays”, SPIE, vol. 634:466-474 (1992)).
While working under the mode of continuous wave, a laser of thermal-conduction cooling type shall have a large block heat sink. Since a passive heat-dissipating mode is employed, this kind of laser tends to have temperature rising, which, in turn, leads to wavelength shift of the laser and reduction in lifetime and reliability of the laser. Consequently, its output power is usually a mere tens of watts. The passive heat-dissipating mode makes it very difficult for the power output of the semiconductor laser to rise from tens of watts to hundreds of watts.
The micro-channel liquid cooling type has currently been in commercial production. Although active heat dissipation is employed in this type of laser to have enhanced the heat-dissipating capability and greatly increase the power output of the laser, it has the following defects:
1. High Costs in Use and Maintenance
A micro-channel liquid cooler needs to use deionized liquid as cooling liquid in order to prevent the positive and negative electrodes from electric conduction. Moreover, low electrical conductivity of the deionized liquid must be maintained throughout the period of use, so that the costs in use and maintenance are very high.
2. Difficulty in Processing
The micro-channel liquid cooler is usually formed by copper materials, which are made by stacking several very thin layers of copper sheets one on the other. The inner diameter of the micro through-channel is about 300 microns. During the processing the micro-channel liquid cooler, each layer of copper sheets should be finely processed, so as to create a turbulent flow with high heat-dissipating capability when the liquid flows through the stacked micro through-channel. But, the fine processing of the micro-channel cooler is a difficult task.
3. High Production Cost
Since the fine processing of the micro-channel cooler is considerably difficult, the production cost thereof is correspondingly very high.
4. Short Lifetime
During the process of operation of the laser, if impurities are present in the cooling medium (which is usually deionized liquid), these impurities tend to attach on the inner wall of the micro channel. On the one hand, particles of these impurities might block the liquid passage of the micro-channel cooler to thereby reduce the cooling effect, thus generating relatively severe heat concentration that leads to shift of the output wavelength of the laser, broadening of the spectrum, reduction in performance reliability and lifetime, and in extreme case, even to burn out the laser. On the other hand, particles of these impurities would cause electrochemical erosion of the wall of the micro channel, and might erode away the wall of the micro-channel cooler in some cases, thus severely affecting the safety of the laser. All these seriously and adversely affect the lifetime of the laser.
5. High Requirement on Sealing
Since the cooling medium flows in a very restricted space inside the micro-channel cooler, undesired pressure decrease is easily generated, whereby flow resistance of the cooling medium is high and sealing is difficult.
In view of the above problems, it is apparent that currently available liquid-cooled lasers as those discussed above are yet to be improved due to inconveniences and deficiencies in structure, in manufacture, and in use. To solve the aforementioned problems in the prior-art liquid-cooled lasers, manufacturers concerned tried their best to research for better solutions, but none of the designs, which have been developed and completed, has been found applicable until now for long. Solutions to these problems have been eagerly sought by those concerned. Therefore, it is indeed one of the important tasks of research and development, as well as an objective in urgent need for improvement in the art, to design a practical, conveniently maintainable, structurally simple, manufacture cost-effective liquid-cooling laser.
In view of the aforementioned defects present in currently available liquid-cooling lasers, the present inventor based on his experiences and professional knowledge acquired through years-long design and fabrication of such products, actively studied, constantly innovated and applied relevant theories, tried to create a novel-structured, and improved liquid-cooling laser with practical applicability. After incessant researches, designs, repeated trail productions of samples, and improvements, the present practically valuable invention has been finally made.